A method for fabricating a semiconductor device is described in U.S. Pat. No. 5,792,679, which application is incorporated herein by reference. This method includes implanting germanium atoms in a silicon substrate and forming a buried region of enhanced germanium concentration. The germanium atoms are implanted to a predetermined depth inside the silicon substrate such that a silicon layer remains on top of the germanium implanted region. Later on, the silicon layer is oxidized on top such that the germanium-containing region is kept unoxidized. The resulting structure is further processed in order to fabricate a field effect transistor. The channel of the field effect transistor includes the germanium-containing region and therefore exhibits a carrier mobility greater than the carrier mobility of pure silicon. This is due to the fact that germanium generally exhibits higher electron and hole mobilities than silicon.